The invention relates to double-action antenna structures suitable in particular for mobile stations, in which structures one component is a retractable whip element.
In the field of portable radio equipment, mobile stations in particular, the manufacture of antennas has become demanding. As new frequency bands are introduced, an antenna often has to function in two or more frequency bands. When the devices are small, the antenna, too, must be small; preferably it is placed inside the casing of the apparatus, thus avoiding an impractical protrusion. Understandably, however, the radiation characteristics of an internal antenna are weaker that those of an external antenna. Moreover, an internal antenna is more sensitive to the effect of the hand of the user, for example. These disadvantages can be reduced using a double-action antenna so that a movable antenna element belonging to the structure can be pulled partly out when necessary in order to improve the quality of the connection.
A retractable whip element is well known as such. If the antenna structure additionally comprises a second radiating element, it is usually an element outside the casing of the apparatus, considerably shorter than the whip element. Such a double-action antenna, which in one operating state is located completely inside the casing of the apparatus, is disclosed in an earlier patent application FI991359 by the same applicant. The structure is depicted in FIG. 1. It comprises a ground plane 110, radiating planar element 120, feed conductor 102 and a short-circuit conductor 103, which constitute the PIFA (Planar Inverted F Antenna) type portion of the whole antenna, located inside the casing of the radio apparatus. The planar element 120 has a slot 121 in it, which is shaped such that the resonance frequency of the planar antenna is as desired. The structure further includes a whip element 130, at the lower end of which there is a connecting piece 131. When the whip is in its lower position, it has no significant coupling with the PIFA parts. When the whip is in its upper position, the connecting piece 131 is in galvanic contact with the planar element 120 on both sides of the slot 121 so that the slot becomes short-circuited. Short-circuiting the slot considerably increases the resonance frequency of the planar antenna, whereby the planar antenna will not function as an antenna in the operating frequency band when the whip is in the pulled-out position. The whip element is so dimensioned that it will function as a monopole antenna in the same operating frequency band, thereby replacing the internal planar antenna. The task of the planar element 120 is then to function as a part in the feed line of the whip and as an impedance-matching element of the whip. The PIFA may also be arranged to have two frequencies so that in its upper position the whip element changes e.g. the lower resonance frequency of the PIFA in such a manner that only the pulled-out whip functions as the radiating element at the lower operating frequency. Then the conductive plane of the PIFA functions as the radiating element at the upper operating frequency. Alternatively, the pulled-out whip element just makes the operation of the antenna more efficient at the lower operating frequency without changing the resonance frequency of the PIFA.
It is an object of the invention to provide a double-action antenna in a novel and more advantageous manner than in known structures. The antenna structure according to the invention is characterized by what is specified in the independent claim 1. Some advantageous embodiments of the invention are presented in the dependent claims.
The basic idea of the invention is as follows: An antenna structure comprises e.g. a PIFA-type antenna located inside the casing of a mobile station, a coupling element and a whip element movable in relation to the former two. The coupling element is a relatively small conductive plane between the radiating plane and ground plane of the PIFA. When the whip element is retracted, it has no significant coupling with the PIFA parts. When the whip element is extended, its lower end is brought into galvanic contact with the coupling element, whereby a significant electromagnetic coupling is established by means of the coupling element between the whip element and the radiating plane of the PIFA. Thus the whip element is fed through the PIFA without a galvanic contact with it. In addition, the coupling element provides for the matching of the whip element. The internal antenna may have one or more frequency bands. In the case of a dual-band antenna, for example, the extended whip improves the operation of the antenna structure in both bands of the internal antenna.
An advantage of the invention is that in the structure according to it the internal and external antenna can be designed and optimized relatively independently. This is due to the fact that the design of the internal antenna need not take into account the matching of the whip antenna when the matching is realized by the coupling element. Another advantage of the invention is that the structure according it is relatively simple and inexpensive since there is no need for separate mechanical parts or components for the matching. A further advantage of the invention is that the structure according to the invention decreases the size of the internal antenna. This is because the coupling element which is placed under the outer end, as viewed from the short-circuit point, of the radiating plane, causes additional capacitance and, hence, decreases the physical size in relation to the electrical size.